This invention is concerned with the inspection and maintenance of pipelines. Over land areas pipelines are usually buried under a three or four foot overlay or, where ground conditions preclude burying the line, as for example in frigid zones or over rivers, it is supported aboveground on spaced support structures. Pipelines conveying materials underwater are usually protectively coated, as for example with concrete, and are submerged to lie on the lake or seabed.
Whatever the environment of the pipeline, the risk of pipeline failure is ever present and the consequences of such failure are potentially grave. As a result, it is desirable to inspect the pipeline as it is installed and thereafter to perform regular inspections to detect any weaknesses prior to failure.
It has been generally recognized that effective inspection of pipeline from the outside of the line is not possible because of the necessarily great spacing between the sensor used for the inspection and the line, because of the large background fields produced by the mass of the line and by earth and because of the impracticability of passing an external sensor over the line. Thus the most widely accepted inspection technique is to pass a pig through the pipeline, the pig having one or more instruments for detecting such weaknesses as areas of corrosion, pitting or out-of-round conditions.
The pig may be driven through the pipeline by fluid flowing in the line or it may be carried by a crawler which includes drive means. Whichever method is used it is common to have the instruments coupled to a recording device as for example a pen recorder which produces a trace related to some characteristic of the pipeline. As the pig is recovered, the record is inspected and suspect areas of potential weakness identified. It then remains physically to locate the suspect area, assess its significance and, if need be, repair it.
Since the rate of progression of the pig through the pipeline is rarely, if ever, constant, the trace bears little relationship to the geographical location of the suspect area. Odometers of various kinds have been associated with the pigs in attempts to correlate the pig signals with the geographical location of the suspect area indicated by those signals but the odometers are in many instances inherently inaccurate in this environment or quickly become worn or maladjusted. Thus this solution has not proven entirely satisfactory. The problem has been exacerbated by the fact that the distances recorded by the odometers are relatively large and by the fact that the pipeline does not usually follow a rectilinear path. Thus the length of pipeline measured by the odometer is not necessarily related to the distance covered by the pipeline.
A common pig is one which examines magnetic characteristics of the pipeline and moves sensors along the interior wall of that pipeline. The device produces a signal proportional to the flux changes in the field produced by the pipeline and a skilled reader of the trade produced by the instrument can, in some instances, identify the nature of conditions in the pipeline causing anomalies in that trace. The girth welds between adjacent sections making up the line can also be identified, because there is a change in the metallurgical characteristic of the wall produced during the welding process or because wall thickness changes or because of the surface roughness of the weld. Pipeline fittings can also be detected, Other types of instruments can also identify some of these characteristic features. Thus a count of the number of girth welds can give a general indication of the geographical location of a suspect part of the line which must be physically inspected. However, because the lengths of the sections making up the pipelines are not constant, because of the large distances involved and of course because the pipeline does not follow a rectilinear course commonly an error of as much as several hundred feet may occur.
Thus with submerged or buried pipeline the technique which has been adopted involves identifying on the trace a specific feature of the pipeline such as a short pipe section or a fitting closest to the suspect area which must be inspected and measuring the distance between that feature and the suspect area as best one can from the trace either by counting girth welds or by the use of an odometer which will be reasonably accurate over the relatively short distance involved. From a map of the line one may then approximately locate that feature. The buried line is then excavated, positively to locate the feature and having made that location the distance to the suspect area may be measured off and that area excavated.
Besides being a time consuming and hence costly procedure, there is a substantial element of risk involved in making these two excavations. The excavations are usually made with a hoe or other mechanical digger and the operator may easily err and damage the pipeline even to the extent of rupturing it.
With a submerged line it becomes necessary to send down one or more divers to uncover and locate the feature and thereafter the diver or divers must move to the suspect section and uncover it.
The present invention seeks to provide an inspection and maintenance method to alleviate the disadvantages of the existing methods.
It is recognized that the sections of pipe which are used to form the pipeline become magnetized to a limited extent during the manufacturing or handling processes. Perhaps because of the opinion that the sections when jointed together would lose their magnetism or that joining them would have the effect of forming a single bar magnet of length equal to the overall length of the pipeline, no value or significance has been attributed to this characteristic. On the contrary, it has been considered to be a nuisance by welders joining the sections end to end when their welding rods have been attracted to and have adhered to the ends of the sections and research temporarily to degauss or demagnetize the end portions of sections of pipe to facilitate welding has been undertaken by others.
For whatever reason, universally it has been reckoned that the anomaly produced at such welds could only be observed by using an inspection pig based on magnetic principles.
I have now discovered that not only do the pipe sections of a pipeline retain their individual bar-magnet characteristics but also that by using a magnetic field responsive means, which in some instances may be a relatively simple device of only routine sensitivity, one may detect the junction between adjacent sections even from a distance of several feet and through an overlay or in water.
Contrary to other experts belief that the magnetic anomaly representing a girth weld in a buried line could not be detected from aboveground, because the anomaly would be lost in the noise produced by earth field and by the field produced by the mass of the pipeline, I have found that not only can I clearly identify that anomaly but I can also identify other features such as fittings and changes in pipeline metallurgy and wall thickness including some which may be dejects such as large hard spots and severe and extensive corrosion.
Accordingly there is provided according to this invention a pipeline inspection andd maintenance method comprising moving a magnetic field responsive device along the route of a pipeline and observing the output of that device to identify pipeline characteristics.
According to another aspect of this invention there is provided a method of inspecting and maintaining a pipeline which comprises moving a magnetic field responsive device over the course of a pipeline, identifying the location of a specific feature of that pipeline and using that location as a datum from which to locate a suspect area of the pipeline.
Using the technique of this invention in a buried line one would examine a record produced by a pig and identify a suspect portion of the line and the nearest specific or characterising feature of the pipeline such as a short section, a fitting, as for example a drip, or a junction or intersection. The distance between the specific feature and the suspect portion is noted either from a record produced from an odometer on the pig or by counting girth welds as recorded by the pig and thereafter consultation of a map of the line gives an approximate location of the specific feature. The pipeline is then examined in that location by following its course with a magnetic field responsive device such as for example a magnetometer or an inclinometer (dip needle). This examination will reveal the precise location of the specific feature and thereafter it is a simple matter to move from that feature to the suspect portion of the pipeline. This procedure will eliminate the dangerous, time consuming and hence, costly, procedure of excavating the pipe in two locations.
The method is also available for use with submerged pipeline. In such instances a sensor is enclosed within a container suitable for trolling along the route of the pipeline, the position of the container being controlled for example by reference to a depth guage. The position of the pipeline is first determined by crossing the route of the pipeline and placing marker buoys as the magnetic field sensitive device detects the pipeline. Thereafter the container is trolled over the route of the line and some small distance above it and the location of a specific feature of that line positively established.
It has also been observed that the signal produced by the magnetic field responsive device as it passes over a junction between the ends of adjacent pipe sections which are of like polarity is markedly greater than when the device is passed over a junction of which the ends of adjacent sections are of opposite polarity. Thus it is proposed according to this invention to identify the polarity of the ends of the sections as the line is assembled (which can be done for example using a simple compass) and order the sections so that there is a junction of adjacent like poles at regular intervals along the route of the pipeline. Such functions provide a very readily identifiable specific feature of the pipeline.
A typical magnetometer useful in the methods according to this invention is a portable proton magnetometer. A specific model is that available from Barringer Research Limited of Rexdale, Ontario, and which is identified by that company as their Model GM-122. That type of magnetometer comprises a proton rich fluid such as kerosene and means for inducing a field in it. This causes the protons to align along the magnetic field vector and when the field is removed the protons act as elementary gyroscopes and begin precessing around the remaining magnetic field, i.e. that of the earth and, in its application herein, also that of the pipeline. The precession frequency is directly proportional to the field and can be read with a high degree of accuracy.